Method for producing cube-on-face oriented structure in a plain carbon iron



Nov. 3, 1970 T. R. MAGER ETAL 3,537,918

I METHOD FOR PRODUCING CUBE-ON-FACE ORIENTED I STRUCTURE IN A PLAIN CARBON IRON Filed April 25. 1968 2 A2 A2 A2 c B A I FIG. I

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-lo l WITNESSES I INVENTOR MJQILMM/ United States Patent O vania Filed Apr. 25, 1968, Ser. No. 724,061 Int. Cl. (321d 7/02 US. Cl. 148-120 11 Claims ABSTRACT OF THE DISCLOSURE A process is set forth for the production of cube-onface oriented plain carbon iron which exhibits enhanced magnetic characteristics. Typical magnetic data are reported and a method for commercial production is disclosed.

BACKGROUND OF THE INVENTION The present invention relates to a process for producing cube-on-face orientation in plain carbon iron and in particular to a process for producing cube-on-face oriented structure in plain carbon iron sheet which is suitable for use in electrical equipment especially in rotating equipment.

Heretofore, annealed low carbon iron with a near random grain orientation has been used in many magnetic devices such as transformers and small motors. It is known, however, that when certain steels containing about 3 to 3.5% silicon are processed to a fiat rolled product such as sheet or strip in a given manner, the so-called cube-on-edge texture is developed in these materials characterized by having a high degree of orientation in the rolling direction. When magnetized in the rolling direction in the plane of the sheet these materials demonstrate outstanding magnetic characteristics.

Another type of orientation which has been produced in the same materials is that referred to as cube-on-face, designated as (100) [001] grain texture in Miller Indices. In this latter class of materials there are two directions of easy magnetization and hence the properties which are exhibited by these cube-on-face oriented materials are outstanding when measured in the rolling direction and in a direction which is perpendicular to the rolling direction and within the plane of the sheet.

Heretofore, however, the cube-on-edge and cube-onface orientations above referred to were only produced in iron base alloys containing silicon in amounts of up to about 5% silicon. These products are of high magnetic quality and are relatively expensive. While the effect of silicon in improving the resistivity as well as other magnetic characteristics of these materials is well known, nonetheless there are certain applications where such high properties are not required. Consequently, a much cheaper raw material such as low carbon iron sheet and strip has been employed in the past. The annealed low carbon iron exhibits a near random grain orientation acceptable where the better magnetic characteristics of oriented silicon steels were not required. However, the performance of some of the electrical equipment can be vastly improved by the presence of a stress-free cubic texture in the low carbon iron sheet. This orientation may be described as one with a high proportion, preferably at least 70%, of the grain volume having their body-centered cubic lattice oriented so that four of the cube faces are substantially parallel within of the rolling direction, two of these faces also being substantially parallel with the plane of the sheet and the other two being substantially perpendicular to the plane of the sheet and remaining two cube faces substantially perpendicular to both the rolling direction and the plane of the sheet. This orientation has been termed cube-on-face cube texture or defined in terms of Miller Indices as (100) [001].

SUMMARY OF THE INVENTION The present invention contemplates the use of a plain carbon iron containing at least about 99% iron, less than about 0.50% silicon, less than about 0.1% carbon and the balance essentially small amounts of incidental impurities. This composition is cast into ingots in the regular manner following which the ingots are hot worked to an intermediate gauge usually within the range between about 0.020 inch and about 0.250 inch. Following hot working the metal is descaled and thereafter cold worked to finish gauge such cold working preferably being accomplished in two steps without an intermediate anneal, with the total reduction of the cross-sectional area being between about 60% and about 90%. In cold working the material to finish gauge, it is Worked in the longitudinal direction to accomplish a reduction in the cross-sectional area of between about 40% and about Thereafter, and without any intermediate heat treatment the material is rotated within the working plane and the material is then cold worked to finish gauge, such cold working effecting a reduction of between about 30% and about 70%. It is essential that the metal be cold worked in these two directions in order to obtain the desired degree of cube-on-face orientation as will appear more clearly hereinafter.

The finish gauge material is thereafter subjected to an initial heat treatment at a temperature within the range between about 500 C. and about 850 C. for a period of time between about two minutes and about eight minutes. Thereafter, the material is reheat treated at a temperature between about 850 C. and the Ac temperature of the metal which in no case exceeds 910 C., the temperature at which the allotropic transformation of pure iron occurs. This latter heat treatment continues for a period of between about four and about seventy-two hours or until the desired degree of secondary recrystallization has been effected. By employing the process as above described, it has been demonstrated by the etch pit technique that over 70% of the grains have a [001] texture developed.

It is an object of the present invention to provide a process for producing (100) [001] texture in plain carbon iron.

It is a more specific object of the present invention to provide a process for producing cube-on-face orientation in plain carbon iron which exhibits improved magnetic characteristics and which is suitable for use in rotational power equipment and transformers.

Other objects of this invention will become apparent to one skilled in the art when read in conjunction with the following description and the drawing, in which:

FIG. 1 is a schematic illustration of the commercial embodiment of the process of the present invention.

FIG. 2 illustrates a step in the cold reduction to finish gauge employing the process of the present invention.

FIG. 3 is another embodiment of the process of the present invention; and

FIG. 4 is a plan view of the finished strip of FIG. 3.

PREFERRED EMBODIMENT The process of the present invention contemplates the use of a material classed as plain carbon iron containing at least 99.0% iron and preferably about 99.5% iron. In addition to the iron content there is also present less than about 0.50% silicon, less than about 0.10% carbon, less than about 0.03% manganese and balance essentially incidental impurities.

The metal may be made in any one of the well known manners for example, the basic open hearth practice, the electric furnace practice or the basic oxygen furnace practice. Preferably, the material has as low a carbon content as is resonable, commensurate with good overall economics and is thoroughly deoxidized, killed or vacuum degassed. The metal is preferably cast into ingots in the usual manner which are thereafter soaked and after being raised to a sufficient temperature for equalization of the heat content the ingots are preferably hot-rolled in manners well known in the art. It is preferred to finish the hot-rolling at a given intermediate gauge, that is, a gauge varying between about 0.020 inch and about 0.250 inch in thickness. The selection of the intermediate gauge is dependent upon the desired final gauge and the cold work to be applied as will be discussed more fully hereinafter.

During the hot-rolling and the cooling from the hotro ling temperature the material devolops an oxide scale and consequently it is desirable to descale the material prior to the commencement of cold working. The descaling may take any form, for example, wheelabraiting, pickling, shot blasting or any other suitable means. It is essential to remove the scale so that the same does not contaminate the metal during the subsequent cold working steps. Such rolled in scale has the effect of impeding the orienation process which is responsible for the improved magnetic characteristics as will be demonstrated more clearly hereinafter.

After the material has ben descaled it is submitted to a cold reduction and, preferably, the cold reduction is in the form of cold-rolling. The metal is preferably reduced in cross-sectional area a total amount of between about 60% and about 90% to finish gauge without any intermediate heat treatment. The iron, preferably in coil form, is initially rolled in the longitudinal direction a sufficient amount to accomplish a reduction in the cross-sectional area of between about 40% and about 80%. This reduction in the cross-sectional area is accomplished without any intermediate annealing and may be done on either a reversing mill, a tandem mill or in any other suitable manner. It is clear, however, that this first cold reduction must not produce the steel of the finish gauge. Instead, an intermediate reduction is taken, following which the metal is rotated in the rolling plane through an arc of 90 so that the rolling direction occurring during the first cold reduction is now perpendicular to the final cold reduction yet maintaining the same in the rolling plane of the sheet. The metal is thereafter cold-rolled to finish gauge and such cold-rolling effects a reduction of between about 30% and about 70% in the cross-sectional area of the steel of intermediate cold-rolled thickness. It is imperative that the steel be rotated said 90 and a minimum of a 30% reduction of the intermediate cold-rolled thickness effected to the steel in the cross direction. Amounts in excess of 70% do not appear to have any additional effect of improving the orientation which is later developed by the material.

The iron of finish gauge is thereafter subjected to its first heat treatment which is preferably accomplished in a protective atmosphere at a temperature within the range between about 500 C. and about 850 C. Good results have been obtained where the time that the steel is maintained at temperature ranges between about two minutes and about eight minutes. It is desired to maintain the iron at this temperature and for this time period while preferably within a hydrogen atmosphere. While other atmospheres inert to the iron may be employed care should be taken so that the atmosphere does not contain such elements as nitrogen or carbon which could react with the iron to form an aging component.

Following the initial heat treatment the material is reannea ed at a temperature within the range between about 850 C. and the Ac temperature of the metal. While it is desired to maintain the iron at a temperature as high as possible, nonetheless it cannot be heated to a temperature sufficiently high that the austenitic phase is formed. Consequentl depending upon the chemical composition of the metal, the upper limit of the final annealing temperature is set at the Ac temperature. The heat treatment is continued for a period ranging between about four hours and about seventy-two hours, the time period being selected in such a manner so that the iron undergoes a complete secondary recrystallization. During this extended heat treatment it is preferred to employ a protective atmosphere, particularly in hydrogen gas. It is during this heat treatment that the cube-on-face orientation is achieved and the iron develops its outstanding magnetic characteristics.

In order to more clearly demonstrate the process of the present invention, reference may be had to the drawings which show one commercial embodiment of a practical manner in which the process can be employed. Referring to the drawing and FIG. 1 in particular there is shown therein a strip of metal generally designated as 10. This is a finite length of iron and usually is processed in coil form. The strip 10 in coil form in its descaled condition was subjected to an initial cold reduction which produced such strip and predetermined segments marked A, B, C and so forth were marked and sheared at 12 to provide a common edge.

At this juncture, each individual segment A, B, C is rotated so that the edge 12 is in a common line as more clearly illustrated in FIG. 2. Each segment is there after rolled so that the edge 12 is parallel to the rolling direction and the final reduction of between 30% and 70% of the cold-rolled intermediate gauge is therea ter effected to the material. Thus segment A, segment B and segment C are individually rolled to finish gauge.

As an alternative, reference is directed to FIG. 3 where in the segments A, B, C are butted together in the manner illustrated. Thereafter, the materials are welded along the butt joint 14 so that the former shear line 12 now provides the common edge in the longitudinal direction. As thus welded the material 10 is subjected to the final cold reduction for the required amount to produce the material of finish gauge 10' as illustrated in FIG. 4. Since the weld metal employed in forming the joints and in the heat affected area adjacent the welds will not possess the impetus to exhibit a secondarily recrystallized structure having a cubic texture, these areas may be later removed from the material or punchings made only from the non-weld portions before it is employed in such finished products as rotational power equipment and transformers.

Example I An ingot of commercial grade pure iron was employed and had the following composition:

Percent and the balance iron with traces of incidental impurities. This ingot was hot-rolled at a temperature of 1000 C. to a thickness of 0.063 inch. The material emerged from the hot-rolling with a scale thereon and as a result the metal was pickled in nitric acid followed by a pickling in sulfuric acid. Thereafter the metal was rinsed and dried in the usual manner. The pickled material was thereafter cold-rolled to a thickness of 0.025 inch. This cold-rolling effected a reduction in the cross-sectional area of 60% of the hot-rolled thickness. The material was then cross rolled to a finish thickness of 0.013 inch. The cross rolling took place in a direction 90 to the first cold-rolling direction and in the plane of the strip. The reduction from 0.025 inch to 0.013 inch effected an additional coldrolled reduction of 48%. Following the cold-rolling to finish gauge the material was subjected to annealing heat treatment at a temperature of 800 C. for a time period of about four minutes in a hydrogen atmosphere. Following the initial heat treatment the material then received the final heat treatment at a temperature at about 880 C. for a time period of 66 hours. This final heat treatment Was accomplished in a vacuum of less than about one micron. Analysis of samples thus processed indicated that the material had undergone a secondary recrystallization, and employing the etch pit technique, the material illustrated that over 70% of the grain volume had a (100) [001] texture within In order to more clearly demonstrate the outstanding success of the process of the present invention, reference is directed to Table I which illustrates the AC core losses of this material having at least 70% of its grain with the cube-on-face texture. The material was tested in each of the rolling directions which in the cube texture material were the easiest directions of magnetization.

TABLE I [AC core loss, 60 Hz.]

Core loss, Core loss.

Flux density, B. (kilogauss) watts/# [001] watts/# [010] Referring to the table it is clear that the core losses exhibited by this material are outstanding at low flux densities, that is, at densities of up to about 10,000 B. At higher flux densities, it is noted that very little difference exists in the core loss exhibited in the rolling direction from that exhibited 90 to the rolling direction in the plane of the sheet. The similar levels of magnetic characteristics is indicative of the cube-on-face orientation. Comparing, however, this material with the commercially available compositions presently on the market it is seen that the core loss properties demonstrated by this plain carbon iron are outstanding in comparison with nonoriented materials having up to 3% silicon contained therein. The metals produced by the process of the present invention have core loss characteristics equivalent to the AISI type M22 steel.

It is preferred when employing the process of the present invention to utilize a plain carbon iron having the lowest possible carbon content commensurate with good economic practice. It will be appreciated however that higher carbon content can be employed and thereafter the material may be subjected to a decarburizing anneal, the same preferably occurring between the hot-rolling and the commencement of the cold-rolling step, such decarburizing anneal being well known in the art. Since the process of the present invention produces outstanding magnetic characteristics, the material produced thereby, havingthe cube-on-face orientation, is especially suitable for use in rotating power equipment and in transformers. This is specifically true where low flux densities are encountered.

We claim as our invention:

1. In the method of producing a plain carbon iron in which at least 70% of the grain volume exhibits an orientation defined in terms of Miller indices as (100) [001] within :10", the steps comprising, selecting a composition of at least 99.0% iron, less than about 0.10% carbon, less than about 0.50% silicon, less than about 0.30% manganese and the balance essentially incidental impurities, hot-rolling said iron, descaling, cold roll reducing said iron in a given direction to effect a reduction in the cross-sectional area of between about 40% and about 80%, rotating said cold roll reduced material 90 within the rolling plane, continuing said cold-rolling in a direction perpendicular to said first mentioned cold-rolling direction to effect a reduction of between 30% and 70%, heat treating said cold roll reduced material at a temperature within the range between 500 C. and about 850 C. and thereafter reheat treating the material at a temperature in the range between about 850 C. and the Ac temperature of the material.

2. The method of claim 1 in which the total cold reduction effected to the material is within the range between about 60% and about 90% in cross-sectional area.

3. The method of claim 1 in which said heat treatment takes place in a hydrogen atmosphere.

4. The method of claim 1 in which the first heat treatment after the iron is cold reduced to finish gauge occurs for a time period within the range between about two minutes with the longer times associated with the lower temperatures and the shorter times associated with the higher temperatures.

5. The method of claim 1 in which the final heat treatment takes place for a time period within the range between about four hours and about seventy-two hours to obtain secondary recrystallization.

6. In the method of producing cube-on-face orientation in plain carbon iron, the steps comprising, selecting a material in which the iron content is at least about 99%, less than about 0.50% silicon, less than about 0.1% carbon and the balance essentially impurities, hot-rolling the metal to an intermediate gauge, descaling the metal, cold roll reducing the metal to effect a total reduction of the cross-sectional area within the range between about 60% and about 90% to finish gauge the last portion of the cold reduction being at a minimum amount of about 30% in cross-sectional area and being effected in a direction perpendicular to the initial cold reduction, heat treating cold reduced metal at a temperature within the range between about 500 C. and about 850 C. and thereafter reheat treating the steel at a temperature within the range between about 850 C. and the Ac temperature of the steel.

7. The method of claim 6 in which the initial cold reduction effected to the metal is within the range between about 40% and about of the cross-sectional area of the starting material.

8. The method of claim 6 in which the final cold reduction effected to the metal is Within the range between about 30% and about 70% in cross-sectional area.

9. The process of claim 6 in which all anneals take place in a protective atmosphere which is non-reactive with the metal.

10. The method of claim 6 in which the first heat treatment after the metal is cold reduced to finish gauge occurs for a time period within the range between about two minutes and about eight minutes with the longer times associated with the lower temperatures and the shorter times associated with the higher temperatures.

11. The method of claim 6 in which the final heat treatment takes place for a time period within the range between about four hours and about seventy-two hours to obtain secondary recrystallization.

References Cited UNITED STATES PATENTS 3,058,857 10/1962 Pavlovic et a1 148-12 3,266,955 8/1966 Taguchi et al. 148-111 L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner US. Cl. X.R. 148-12, 121 

